1. Field of the Invention
This invention relates to electrostatic actuators suitably used as driving sources of shading devices such as vehicular sunvisors, moon-roofs, and rear-windows, or those for trains, aircraft, ships or others, or paper-feeders for copying machines. More particularly, it relates to simplification in structure and prevention of electrification of a stator element and a movable element of an electrostatic actuator which has electrodes provided on respective insulation layers of the stator element and the movable element so that the movable element is driven by attractive force and repulsive force resulting from static electricity created between the electrodes of the stator element and the electrodes of the movable element.
2. Description of Related Art
Conventionally, as an electrostatic actuator of this kind, there has been an electrostatic actuator, shown in FIG. 40, previously proposed in Unexamined Utility Model Publication (Kokai) No. H7-16599 by the present applicant.
In this electrostatic actuator, the movable element 1 has electrodes 3a, 3b, 3c . . . 3a, 3b, 3c . . . provided in an insulation layer 2 thereof so that electrodes 3a, 3b, 3c positioned every third one of these electrodes 3a, 3b, 3c . . . 3a, 3b, 3c . . . are connected so as to provide three phases, or a-phase, b-phase, and c-phase. With the a-phase electrode 3a and the b-phase electrode 3b are further connected together to form two phases. On the other hand, the stator element 4 has electrodes 6a, 6b, 6c provided in an insulation layer 5 thereof so that electrodes 6a, 6b, 6c positioned in every third one of these electrodes 6a, 6b, 6c are connected together to provide three phases, or U-phase, V-phase, and W-phase. In this manner, where the electrodes 6a-6c of the stator element 4 are structured in three phases, electrodes of two phases, or the U-phase and V-phase electrodes 6a, 6b in FIG. 41, of the U-phase, V-phase, and W-phase electrodes 6a-6c are connected to electricity-feed portions 7A, 7B provided in a plane same as the insulation layer 5 in which electrodes 6a-6c are arranged, while the remaining one phase of electrodes, or the W-phase electrodes 6c in FIG. 41, are connected to an electricity-feed portion 7C located in a surface of the insulation layer 5 opposite to the surface provided with electrodes 6a to 6c through a conductive member 8 inserted in a through-hole 5a formed in the insulation layer 5.
In this structure, however, there is a necessity of providing an electricity-feed portion 7C on a side opposite to the electrodes 6a-6c and the electricity-feed portions 7A, 7B, as well as providing the through-hole 5a and the conductive member 8, resulting in structural complexity for the stator element 4.
On the contrary, Unexamined Patent Publication (Kokai) No. H2-311186 provides an electrostatic actuator, as shown in FIG. 42, which possesses three phases of electrodes 11a, 11b, 11c provided in the stator element 10 so that one phase of electrodes 11b are arranged in a zigzag form with respect to other two phases of electrodes 11a, 11c, thereby providing three phases of electrodes 11a-11c in one surface of the stator element 10.
However, in the electrostatic actuator of Unexamined Patent Publication No. H2-311186, shown in FIG. 42, the stator element 10 is complex in structure because there is a necessity of providing three phases of electrodes 11a, 11b, 11c with one phase of electrodes 11b arranged in the zigzag form, as stated hereinbefore. Also, in this electrostatic actuator, there is a problem that the driving force for the movable element is small because voltage is adapted to be applied solely to the electrodes 11a, 11b, 11c of the stator element 10 without applying voltage to the electrodes of the movable element.
Also, Unexamined Patent Publication No. H4-281371 discloses an electrostatic actuator having, as shown in FIG. 43, electrodes of three phases of R-phase, S-phase, and T-phase in the stator element, wherein an S-phase electrode is arranged zigzag relative to other two phases of electrodes, to provide three-phase electrodes in one surface of the stator element with the spacing unequally made between electrodes of three phases.
The electrostatic actuator of Unexamined Patent Publication No. H4-281371, shown in FIG. 43, also has the electrodes of three phases provided in the stator element so that the stator element is complex in structure. Further, the driving force is small because of absence of electrodes in the movable element.
Further, Unexamined Patent Publication No. H5-176558 proposes an electrostatic actuator, which as shown in FIG. 44 has electrodes of four phases a, b, c, and d provided zigzag in the stator element with the spacing made unequally between electrodes of four phases.
In the electrostatic actuator of Unexamined Patent Publication No. H5-176558, shown in FIG. 44, the stator element is particularly complex in its structure because four phases of electrodes are provided in one surface of the stator element. And no electrodes are provided in the movable element, not shown, resulting in small driving force.
Furthermore, Unexamined Patent Publication No. H4-236179 proposes an electrostatic actuator, which as shown in FIG. 45 has a structure of two phases provided for both of electrodes 13a, 13b of the movable element 12 and electrodes 15a, 15b of the stator element 14.
In the electrostatic actuator of Unexamined Patent Publication No. H4-236179, the electrode pitch is constant for the electrodes 15a, 15b of the movable element 14 as well as the electrodes 13a, 13b of the stator element 12, to provide such a structure that the movable element is driven solely by the attractive force acting on between the electrodes 15a, 15b of the movable element 14 and the electrodes 13a, 13b of the stator element 12, reducing the driving force small. Also, in this electrostatic actuator, there is a necessity of switching over of the polarity of voltages applied to both of electrodes 13a, 13b of the stator element 12 and the electrodes 15a, 15b of the movable element 14, imposing complexity in its control manner.
Further, in the above conventional electrostatic actuators, there may be a case where voltages with a waveform in the same polarity be applied to a plurality of phases of electrodes present in the movable element or stator element. In the electrostatic actuator shown in FIG. 40, for instance, there is a state in which voltages in the same polarity "+" are applied to the U-phase electrodes 6a and the W-phase electrodes 6c as shown in the figure, though switching over is made for the polarities of the voltages applied to the electrodes 6a, 6b, 6c of the stator element 4. In such state, there is a possibility of electrification of "-" on the insulation layer 5 of the stator element 4.
Also, if there occurs deviation in the polarity of voltage applied to electrodes of one phase among plural phase electrodes within one period, there may occur electrification on either the movable element or the stator element which is provided with the electrodes. For instance, if the polarity of voltage applied within one period involves deviation toward the "-" side in one-phase electrodes, there is a possibility that electrification occurs on either the movable element or the stator element which includes electrodes. This electrification on the movable element or the stator element may possibly lower the driving force for the movable element.